A long-standing challenge for welfare economics is to develop welfare criteria that can be applied to allocations with different population levels. Such a criterion is essential to resolve the optimal population problem, i.e., the tradeoff between population size and the welfare of each person...

This paper extends Harsanyi's Impartial Observer Theorem by introducing Knightian Uncertainty in the form of individual belief systems. It features an axiomatic framework of societal decision-making in the presence of individual uncertainty. The model allows the analysis of scenarios where...